1. Field of the Invention
The present invention relates broadly to systems for measuring magnetic fields using flux licked loops and superconducting quantum interference devices. More particularly, the present invention concerns a system comprising an unmodulated or direct-feedback flux locked loop electrically connected by first and second unbalanced coaxial transmission lines to a superconducting quantum interference device.
2. Description of the Prior Art
Superconducting quantum interference devices (SQUIDs) are small, cryogenically-cooled magnetic field sensors comprising a ring of superconducting material interrupted by two Josephson junctions. SQUIDs are designed to detect changes in magnetic flux, and, when suitably biased with a small DC current, will exhibit a magnetic flux sensitivity noise floor of approximately 1×10−6 0/Hz for low temperature devices that operate near 4 degrees Kelvin (typically cooled by liquid Helium), and approximately 7×10−6 0/Hz for high temperature devices that operate near 77 degrees Kelvin (typically cooled by liquid Nitrogen). SQUIDs exhibit a transfer function that converts magnetic flux into a periodic electrical output signal.
The standard read-out method for SQUID measurements is to inject an alternating current (AC) magnetic field modulation signal into the SQUID and then, using a flux locked loop (FLL) circuit, sense changes in the modulating signal due to external magnetic fields. Without the FLL, the SQUID would have a very limited dynamic range because of its extremely non-linear magnetic field-to-voltage transfer function characteristic. The FLL maintains a stable magnetic flux operating point at the SQUID by introducing a feedback magnetic flux that precisely counteracts the externally applied magnetic field, provided the slew rate and dynamic range of the SQUID and FLL are not exceeded. Measurements of the external magnetic flux can be made by measuring the feedback signal which is an identical image of the external magnetic flux signal within the tracking bandwidth of the FLL.
Also, as the input magnetic signal to the SQUID is varied, the output voltage of the SQUID appears as a distorted sine wave with a period equal to the flux quantum: (φo=h/2e ≈2femtoWebers), where h is Plank's constant and e is the charge on an electron. Only fields smaller than one-half φo can be uniquely detected because any change in the magnetic field of greater than one-half φo results in a nonmonotonic (multivalued) output signal. This small limiting field strength provides little dynamic range and has little practical value.
Systems using SQUIDs for non-destructive testing/evaluation of materials or structures or for making biomagnetic measurements were long impractical for use in field settings (i.e., environments containing high levels of magnetic interference). The prior art had been limited to a flux modulation frequency of approximately 500 kHz with a maximum tracking loop bandwidth of 250 kHz. In magnetically unshielded environments, large amplitude or high slew rate external stray magnetic fields from 50/60 Hz AC power lines, AM broadcast transmitters, small changes in the Earth's magnetic field, and other sources, caused the FLL to lose lock and thereby invalidate any measurement in progress. Furthermore, the prior art employed traditional twisted-pair wires which were highly undesirable for several reasons, including that they had a high degree of linear attenuation versus frequency that severely distorts square waves of even moderate frequencies, they allowed for a large amount of radiated leakage and corresponding susceptibility to radio-frequency interference, and they had a highly variable characteristic impedance that changed with mechanical stress and was difficult to impedance match. The incorporation of digital signal processing (DSP) technology into the FLL had been attempted with limited success due to inherent delays associated with signal acquisition, processing and reconstruction of the feedback signal, and the maximum clock frequency of the DSP. Because of these problems, early attempts to incorporate DSP into the FLL failed to increase the operating frequency above that obtainable with standard analog read-out systems. For these reasons, SQUIDs were restricted to use in controlled environments shielded from magnetic interference, and were typically expensive, bulky, and non-portable.
A great many of these limitations and disadvantages were overcome by the improvements and advances disclosed in U.S. Pat. Nos. 6,420,868; 6,448,767; and 6,356,078 (the '868, '767, and '078 patents, respectively). More specifically, the '868 patent discloses read-out electronics incorporating innovative circuit designs that extend the frequency of operation of the FLL and improve upon the earlier prior art by a factor of at least ten, thereby making operation of the SQUID practical in unshielded environments by alleviating the effects of high levels of magnetic interference on SQUID measurements. The '868 patent also discloses replacing traditional twisted-pair wires with shielded, unbalanced, controlled-impedance transmission lines to overcome many of the problems encountered in the earlier prior art, including reducing the amount of radiated leakage and corresponding susceptibility to radio-frequency interference. The '868 patent also discloses employing DSP algorithms to filter, extract, and measure the weak SQUID output signal. Problems encountered in earlier attempts to incorporate DSP technology into SQUID read-out electronics were overcome by locating the DSP outside of the FLL.
The '767 patent discloses implementing the FLL with analog and radio-frequency (RF) components to improve upon the earlier prior art by a factor of at least ten. The use of RF techniques results in a flux modulation frequency of at least 33 MHz and a maximum tracking loop bandwidth of at least 5 MHz. The FLL is thus able to track, without unlocking, undesired high slew rate magnetic interference, thereby further eliminating the need for expensive and restrictive magnetic shielding for the SQUID.
The '078 patent discloses a system with continuous signals and no time switching devices and therefore none of the associated problems found in the earlier prior art. The '078 patent also discloses operating a plurality of RF FLLs and their associated SQUIDs on different flux modulation frequencies (f1 through fN). This allows for a 1×N architecture which reduces from 2N to N+1 the number of required cable connections between the cryogenic SQUIDs and their associated room temperature read-out electronics. Thus, for example, a system comprising an array of ten SQUIDs, which previously would have required at least twenty cable connections, with their associated heat transfer and added complexity, now requires only eleven connections. The '078 patent also discloses reducing redundancy of FLL componentry by sharing certain components among the SQUID subsystems, thereby reducing complexity, cost, and size of the system as a whole. Thus, while each SQUID still requires some dedicated, frequency-specific FLL componentry, other non-frequency-specific FLL component functions are performed by shared or common components.
Thus, the '868, '767, and '078 patents greatly improved upon and enhanced the usability of prior art FLLs and SQUIDs. These prior art patents, however, refer to and make use of only modulated FLLs. Unfortunately, modulation is associated with a greater number of electronic components, a greater number of more difficult adjustments, and distortion-producing non-linear RF components such as, for example, modulation oscillators, that emit RF interference. Modulation of the SQUID transfer function can also create unwanted distortion and signal sidebands with high level magnetic field signals applied to the SQUID. Modulated FLLs also require substantial bandwidth to process signal information. Modulated FLLs are also non-linear and therefore require band-limiting RF filters, which results in lower slew rates and narrower tracking bandwidths
Due to the above-identified and other problems and disadvantages in the prior art, a need exists for an improved FLL for use with a SQUID in a system for measuring magnetic fields.